Chironomids, commonly known as midges, are mosquito-like insects whose larvae live in the sediments of all types of aquatic habitats. They are abundant in western Lake Erie and the Detroit River. Swarms of adult midges emerge in the spring and summer. They can often be seen flying around lights on warm summer nights. After mating, females deposit their eggs on the water surface where they sink to the bottom and then hatch to become larvae. There are over a thousand species of chironomids in Canada and the U.S. Some species can complete their life cycle in just a few weeks. The larvae of other species of chironomids can spend up to several months feeding on organic matter in the sediments. Chironomid larvae are an important food source for fish and waterfowl (Ciborowski and Corkum 2003). The adults provide food for amphibians, bats and insect-feeding birds such as purple martins and swallows (Smits et al. 2005).

Chironomids can be an important freshwater indicator. The larvae of some species are sensitive to specific forms of pollution, whereas others are quite tolerant. Because the larvae often feed on the dead plant and animal debris in aquatic sediments, they are exposed to contaminants contained in the organic matter. The fact that chironomids live in such a wide variety of habitats makes them especially useful indicators. Large numbers of pollution-tolerant chironomids are often indicative of poor water quality (characterized by low dissolved oxygen and high nutrient concentrations). These species have a substance similar to haemoglobin in their blood which allows them to survive in places where the oxygen has become depleted. Excellent water quality conditions are often characterized by relatively low densities and high species diversity (50% or more of the species being chironomids). Chironomid species diversity and their sensitivity to eutrophic conditions have been used to create trophic status classifications of lakes (oligotrophic, mesotrophic and eutrophic; e.g., Saether 1975; Winnell and White 1985; Langdon et al. 2006).

The value of chironomids as an indicator pertains to more than just their abundance. Correlations have been found between larval mouthpart and antennae abnormalities and exposure to heavy metals and pesticides such as DDT, DDE, dieldrin and hexacholorobenzene (Warwick 1985; Dermott 1991; Hudson and Ciborowski 1996a; Doherty et al. 1999). Deformities in chironomids are relatively rare (although much more common than many other aquatic organisms, making them a good indicator of contaminant effects), so detecting an increase above the baseline level of deformities may require looking at over 100 larvae per site (Hudson and Ciborowski 1996a; Burt et al. 2003). Midge larvae are able to metabolize organic contaminants such as PAHs (Harkey et al. 1994), but once metabolized these compounds may be responsible for abnormalities in body parts. Research has also shown that sediments contaminated with trace metals and other pollutants harbor chironomids whose chromosomal activity levels are reduced, which could reflect lowered metabolic activity and inhibited RNA synthesis - the critical process by which DNA is translated to proteins (Hudson and Ciborowski 1996b). The important role that chironomids play in the food web is also significant for representing the possible transfer of contaminants (Ciborowski and Corkum 2003; Smits et al. 2005).

Figure 1. Comparison of number of genera and density of chironomid larvae collected from western Lake Erie locations in 1982 (open circles) and 1993 (filled squares). Lower larvae density and more species in 1993 is indicative of higher water quality, which supports recovery of Western Lake Erie to more mesotrophic conditions (intermediate rates of plant growth and decay). Data of Doherty et al. (1999) analyzed from samples provided by D.W. Schloesser, Great Lakes Science Center, USGS.

Figure 2. Mean ±SE density of larvae of Chironomidae in the Detroit River between 1968 and 2004. The number of sites sampled each year is indicated. Data compiled from Thornley and Hamdy (1984), Farara and Burt (1993), Wood (2004), and Ciborowski et al. (2006).

In western Lake Erie between 1930 and 1961, increasing eutrophication (characterized by high levels nutrient levels and low oxygen) was evidenced by a fourfold increase in chironomid density (Carr and Hiltunen 1965). In 1961, the three most abundant and widely distributed groups of organisms were chironomid larvae, oligochaetes and fingernail clams. Chironomids made up 355 larvae per square meter of the total zoobenthic abundance and were evenly distributed at all sites across western Lake Erie (Carr and Hiltunen 1965). There was no correlation with the number of oligochaete worms found, so chironomid larvae represent an independent indicator of environmental condition. Water conditions improved in Lake Erie through the 1980s and into the early 1990s. The benthic community slowly recovered as the western basin of Lake Erie returned from a eutrophic state to mesotrophic status. Doherty et al. (1999) examined the chironomid larvae in samples collected from western Lake Erie by the U.S. Geological Survey in 1982 and 1993. Between those periods of time, mean density declined whereas diversity (number of genera) rose (Figure 1).

In the Detroit River, the overall abundance of chironomids has increased steadily from 1968-2004 (Figure 2).

Deformities

The overall incidence of mouthpart deformities in two genera (Procladius and Coelotanypus) decreased from the 1980s to the 1990s (Doherty et al. 1999; Figure 3). Hudson and Ciborowski (1996a) studied the frequency of deformities in chironomids collected from five locations in the Huron-Erie Corridor in 1992 and 1993. Deformities were most commonly found at the head of the Detroit River near Peche Island. They were surprisingly rare at a highly polluted location in the Trenton Channel, possibly because the larvae that could survive in Trenton Channel sediments were especially resistant to pollutants. Midge larvae reared in Trenton Channel sediments in the laboratory were much more prone to deformities than those reared in sediments with less contaminants (Hudson and Ciborowski 1996b). The incidence of deformities in chironomids collected in the Detroit River in 2004 (J. Zhang, University of Windsor, personal communication) was lower than that observed in 1992 and 1993 (Hudson and Ciborowski 1996a).

Management Next Steps

The relative abundance, community composition and morphological condition of chironomid larvae are all useful indicators of the condition of water and sediments in the Detroit River and western Lake Erie. Time trends suggest that concentrations of deformity-inducing contaminants in Detroit River sediments declined meaningfully between the 1980s and the early 1990s. This reflects the concerted cleanup efforts that were undertaken at that time. Since data does not exist past 2004, it is unknown if the continuing efforts to remediate sediment contamination in the Detroit River has resulted in continued reduction in the incidence of deformities. Increases in midge larval biodiversity (number of genera) also suggest improving water quality in western Lake Erie between the 1980s and 1990s. The trend of increasing mean density of larvae in the Detroit River could imply either improving water quality conditions (improved survival) or declining sediment quality (enriched sediments, which sustain more larvae). This uncertainty could be resolved by examining community composition, or the types of species represented (e.g., Winnell and White (1985).

Research/Monitoring Needs

Chironomids are a dominant part of the benthic community of the Detroit River and western Lake Erie. Because they can be found year-round and live in all types of aquatic habitats, the timing of benthic sampling is not as critical for these organisms as it is for other zoobenthic indicators such as Hexagenia mayflies and caddisflies. However, assessment of deformities requires that adequate numbers of larvae be collected at each location. Consequently, multiple replicate samples should be collected during surveys to assure the availability of enough specimens. Community composition assessment can be a valuable tool, permitting use of species richness or pollution indices to assess changes in water quality or local conditions. Genus level identification requires that larvae be mounted on microscope slides and examined by an expert. However, samples that are properly preserved and stored can be examined and identified many years after they have been collected.

Burt, J., J.J.H. Ciborowski, and T. B. Reynoldson. 2003. Baseline incidence of mouthpart
deformities in Chironomidae (Diptera) from the Laurentian Great Lakes, Canada.Journal of Great Lakes Research 29:172-180.

Carr, J., and J. Hiltunen. 1965. Changes in the bottom fauna of Western Lake Erie from 1930 to 1961. Limnology and Oceanography 10:551-569.

Thornley, S., and Y.H. Hamdy. 1984. An assessment of the bottom fauna and sediments of the Detroit River. Ontario Ministry of the Environment, Southwestern Region and Water Resources Branch, Toronto, ON.

Winnell, M.H., and D.S. White. 1985. Trophic status of southeastern Michigan based on the Chironomidae (Diptera). Journal of Great Lakes Research 11:540-548.

Wood, S. 2004. The use of benthic macroinvertebrate community composition as a measure of contaminant induced stress in the sediments of the Detroit River. M.Sc. Thesis, University of Windsor, Ontario, Canada.